# Flies and windshields

Discussion in 'Off Topic [BG]' started by moley, Oct 23, 2002.

1. ### moley

Sep 5, 2002
Hampshire, UK
I deemed this question important enough to have it's own thread, as Jazzbo closed the other thread .

This is my theory, physicists please correct me if I'm wrong...

It depends on where the fly took off from. If the fly took off from the ground, and entered the car and the windows were subsequently closed, it would have to maintain the same speed as the car in order to maintain the distance from your face. This is because the car is not carrying the fly, as the fly is not resting on any part of the car. If you were to then brake, the fly would hit the windshield, because the car would slow down and the fly wouldn't. As such, iplaybass gave the right reason, but the wrong answer. It's the same principal that says that if you're driving along at the same speed as the car in front, and the car in front brakes and you don't, you hit the car in front, not the car behind - quite simple!

However, if the fly was sitting in the car to begin with, and jumped straight up, no, it wouldn't go shooting backwards, because when the fly jumped, it was already travelling at the speed of the car, because it was being carried by the car. So, when it jumped up, it's vertical velocity increased, but it retained the horizontal velocity it had when it was sitting down. However, because you are moving at the same horizontal velocity as the fly, it looks like he's jumping straight up - because all you experience is the *difference* between the fly's velocity and yours (which is none in this case). Like if you were sitting in a car and threw a ball straight up, to you it would look like the ball was going straight up and down, but to someone standing on the ground as you go past - it would look like the ball was travelling sideways at the speed of the car, as well as up, if you see what I mean.

However, if the fly was sitting in the car to begin with, but then started *flying* (rather than just jumping on the spot), and tried to hover in front of your face, I *think* the fly would actually have to fly at the speed of the car in order to maintain its position. So if you were going at 70mph, the fly would have to be actually flying that fast. I don't think flies can fly that fast I suppose the momentum of the car would help it out initially, if it jumped before it started flying, but as it lost the momentum it got from the movement of the car, it would not be able to keep up the speed, and thus it would hit your face, without you having to brake. Because this time, the fly would slow down, not you.

That's my theory anyway...

2. ### tufnuts

It's called relativity. Study it.

And you've forgotten to take air into account. This part gets iffy. The fly uses the air to maintain it's position relative to the car. The fly's mass plus the dynamics of the air movement will tell you what it'll do. You can theorize all you want, you have to test it to make it solid.

Me? What do I think? Fly will stay in the same position pretty much. It's body/wings will slow down itself when inertia comes into play. But it will move forward.

3. ### Josh Ryan- that dog won't hunt, Monsignor.Supporting Member

Mar 24, 2001
Tufnuts is right. Think about how you can throw an ice cube at your friend in the front seat from the back seat while the car is moving at 85 mph. Did you throw the ice cube at over 85mph? I don't think so.

4. ### tufnuts

If someone was watching you from outside the car, yes you did. Inside, no.

Personally? I throw ice cubes at 85mph anyway

5. ### thrash_jazz

Jan 11, 2002
Artist: JAF Basses, Circle K Strings
It isn't relativity, it's dynamics. I think what Tufnuts was talking about was inertial frames of reference.

In answer to 72beetle's question, the fly will probably not move far when you slam on the brakes. When you stop a car suddenly, what jolts you is the fact that the car has already decelerated, but you haven't. Since a fly is light enough to stay in the air, it will only feel the effects of the air column's movement, which will be considerably less. Since flies are pretty agile in the air anyway, I don't think it'd be affected much at all.

Moley - the fly would not have to fly at 70 mph, per se. It would have to appear to be flying that fast to an observer on the ground, but anyone and anything inside the car can essentially consider the car as its rest frame. This includes people, coffee cups, air molecules, and thus, flies.

6. ### BIg O

Apr 3, 2002
thrash - well put.

Relativity wouldn't be a substantial issue unless the velocity of the car (and thus the fly in the car) vs the earth was significantly higher (i.e. approaching a reasonable fraction of the speed of light).

However, there is an almost insignificant relativistic effect even in this case, although it would not affect the ultimate destiny of the fly. It would, however, affect the age of the fly at the time of the impact if viewed from the perspective of the fly versus his brother who remained stationary on the earth.

Although the difference would be in the fractions of nanoseconds, it seems like a comment I should make to enlighten those not up on relativity and waste some more bandwidth.......

7. ### moley

Sep 5, 2002
Hampshire, UK
Hmmm... well you're right about air, that'd make a difference, but I really couldn't be bothered with factoring that into it Thing about throwing ice cubes - yes, relative to the ground, the ice cube is moving at 85, but relative to the car, it's moving however fast you throw it.

I'd like to see you train a fly to do this

I don't think you can do that... see, the people and the cups and the air, are being carried by the car, and the fly - well, less so, since it's not touching the car. But of course the air will make a difference. So I don't know what will happen, I guess it's all down to the effect of the air. Perhaps the air in the car will carry the fly.

8. ### iplaybass

Feb 13, 2000
Houston, TX
Originally posted by iplaybass
C. The brakes will stop everything attached to the car. You are, the fly is not. To be hovering in front of your face, the fly has to also be traveling at 70mph(relative to the ground). When you slow down, he does not.

I need to correct myself here, I meant to say that the fly will slam into the windshield as you slow down. If you sped up, it would fly into your face. Relativity is the key here. Force is not a result of velocity, rather a result of acceleration(Newton's second law, F=MA). Therefore, to maintain a constant speed when there is no net force acting on you requires no effort on your part. Back to the fly. He is traveling at 70mph relative to the ground, as is the car. If you apply the brakes, everything that is attached to the car in some way will slow down as a result of the force from the brakes. So, the car will drop its speed to say, 50mph. No force has been applied to the fly, so our little buddy is still going 70mph. Splat. The fly is flattened against the windshield by the normal force(force applied by the surface) of the windshield that is being supplied, in a roundabout way, by the brakes. Hope that made sense.

Relativity is the key here: we are all traveling at around 1700mph at the moment due to the rotation of the earth. However, since everything is traveling at this speed, we never notice. The solar system is traveling even faster through the galaxy... and so on and so forth.

EDIT: Think about this experiment in a vacuum. Now think about it in water. And now air. How effective, really, is non-pressurized air at transferring force? Not very. It all comes down to how effectively the medium transmits the force. Solids are very effective at this, obviously. Gases, due to their very nature, are not. They mold and conform and spread out to fill a void.

9. ### iplaybass

Feb 13, 2000
Houston, TX
You've got the right example, but you applied it to the wrong answer. Think of yourself as the fly. The car has decelerated. You haven't.

10. ### redjeep!Guest

Jan 19, 2002
Dublin
You've got to think of it as two frames of reference the first one being the car moving at 70mph (or whatever), the second is the ground which is stationary. You can of course argue that the earth is moving thro' space, but this is fairly irrelevent as everything we measure is relative to it - you've never been pulled over by a policeman who claimed that you were doing 17075 mph have you ? ?

I believe that one of Einsteins postulates was that no frame of reference can take precedence over another (meaning that you have to choose one that's applicable and relevent).

Applying the brakes will slow down the car. Anything within the car will want to continue at 70 mph unless there is some reason why it'll decelerate with the car - for the driver this is probably the friction of you sat on the car seat, so you'll slow down at about the same rate. If this isn't enough (i.e you slammed the brakes on rather than just touched them) , then you'll probably need the safety belt/ sterring wheel/ air bag/ windscreen/ hood / front fender to stop yourself.

Your coffee in your Starbucks cup will decelerate slower (less friction etc) and so will probably slop over the top and you'll be able to sue them if it was too hot (hope it wasn't between your thighs).

The Bud can under the passenger seat will roll forward until it hits something (probably that empty McDonalds bag).

Getting to the point, the fly will carry on at 70 mph, unless it does something to stop itself and will slam into the window. But it's unlikely that the car will be able to slow down so fast that it won't be able to correct itself.

Hope this explains it - I don't feel that relativity has anything to do with it (but it's been a while since I did any real Physics).

Andy

11. ### thrash_jazz

Jan 11, 2002
Artist: JAF Basses, Circle K Strings
True, but IMO it isn't going to go splat against the windshield. The fly's momentum is obviously much less than that of a human, enough so that air resistance will prevent it from going very far. And the movement of the air inside the car would be, I believe, independent of the car's motion.

Who knows... someone needs to try this out to see for sure. Any volunteeers?

12. ### tufnuts

Okay, better way to put it. Dynamics will tell you what happens when acceleration/deformation occurs. It's just the study of energy. Relativity will tell you what direction the fly will go.

Why not make an object the same mass / size as a fly, mount it on a string that runs the length of the cabin of the car, and make it so the fly-thing can move about on it. Then do your test.

No prob.

13. ### thrash_jazz

Jan 11, 2002
Artist: JAF Basses, Circle K Strings
Hate to play devil's advocate, Tuf, but the object wouldn't simulate the fly flying. That's picking at nits, I know, but it might be significant...

14. ### Johnny BoomBoom

Jun 8, 2001
Glasgow, Scotland
Well there's two ways of looking at this - either the fly hits the windshield, or the fly stays hovering next to your face throughout your emergency stop. Who gives a %^&*@"£\$!!!!!!

Only kidding - mods if I went too far, please delete this post!!!

Two ways of looking at this one. The fly is an integral parts of the system - like you are when your strapped into your seat. Or it just happens to be an independent litte so and so within the system! I believe that it is an integral part of the system, and when you slow down the car, everything else decelerates slightly after the event. So much like you would if you weren't strapped in - the fly hits the windshield!

Admittedly, it has been 13 years since I got my Physics degree, and my day job hasn't required me to owkr this one out during that time

Hmmm, this makes me wonder - when you're doing 70 mph in your car, and you switch on the lights, does the light emanating from your lights momentarily travel faster than the speed of light (because the system was already doing 70mph), or is the light from cars slow (ie speed of normal light - 70mph?)

15. ### tufnuts

yup, I know. But you gotta start somewhere. Best idea yet...if I say so myself

16. ### jazzbo

Aug 25, 2000
San Francisco, CA
I don't know if I know of a vehicle that can stop quickly enough to make this an issue.